In the case of a gas and steam turbine plant, the heat which is contained in the expanded working medium or heating gas from the gas turbine is used for generating steam for the steam turbine. The heat transfer is carried out in a waste heat steam generator which is connected downstream to the gas turbine and in which a number of heating surfaces for water preheating, for steam generation and for steam superheating are customarily arranged. The heating surfaces are connected into the water-steam cycle of the steam turbine. The water-steam cycle customarily comprises a plurality, for example three, of pressure stages, wherein each pressure stage can have an evaporator heating surface.
For the steam generator which as a waste heat steam generator is connected downstream on the hot gas side to the gas turbine, a plurality of alternative design concepts, specifically the design as a once-through steam generator or the design as a recirculating steam generator, come into consideration. In the case of a once-through steam generator, the heating of steam generator tubes, which are provided as evaporator tubes, leads to an evaporation of the flow medium in the steam generator tubes in a once-through pass. In contrast to this, in the case of a natural or forced-circulation steam generator, the water which is guided in the cycle during a pass through the evaporator tubes is only partially evaporated. The water which is not evaporated in the process, after a separation of the generated steam, is fed once more to the same evaporator tubes for a further evaporation, wherein the evaporated portion is replaced by water which is fed again to the evaporation system.